Abstract: Disclosed herein are a power control system (110) and method for a light emitting diode (LED) lighting device.

Abstract: A light emitting diode (LED) driving circuit structure includes a surge absorber unit, a voltage suppression unit connected to the surge absorber unit, a rectifier unit electrically connected to the voltage suppression unit, a control chip connected to a fourth junction of the rectifier unit, and an impedance unit connected to the control chip. The LED driving circuit structure is installed on an LED light circuit board having a plurality of LEDs mounted thereon. According to the sinusoidal wave form of the alternating current (AC) supplied to the LEDs as well as the number and layout of the LEDs on the LED light circuit board, the LED driving circuit structure controls the current flowing through the LEDs to vary with changes of supply voltage, so as to enable effectively increased power factor and reduced harmonics, as well as effectively lowered manufacturing and material costs.

Abstract: A physiological monitoring system may use a differential light drive to illuminate one or more light sources. A differential light drive may include applying two signals, one to each terminal of a light emitting diode or other light source, such that the illumination of the light source is controlled by the difference between the two light drive signals. In some embodiments, light emitting diodes may be turned on and off using a differential light drive without using switches and using only unipolar voltage sources. In some embodiments, light drive signals may be 180 degrees out-of-phase, and the phase shift may be used to reduce crosstalk and other electronic noise, for example by carrying the signals in a twisted pair of conductors.

Abstract: A medical light-source device for securing a long illumination time required for operations in the medical field, has an LED illumination section including an LED element; a holder to put the LED illumination section on a head of the operator; a battery power supply section that supplies power to the LED illumination section; a charger having an AC adaptor capable of being connected to a commercial power supply for charging the battery power supply section; and a battery holding belt to put the battery power supply section and the charger on the body of the operator. The battery holding belt has a switch section to switch on/off the LED illumination section and to adjust intensity of illumination of the LED element; and a control section to control a constant current, which is associated with the intensity of illumination, supplied to the LED illumination section.

Abstract: A lighting circuit include a connecting section connected to a light source module to thereby form at least a first path and a second path, a power supplying section connected to the connecting section and capable of supplying first direct-current power and second direct-current power to the light source module, a detecting section configured to detect the connection of the light source module, and a control section configured to determine, when the detecting section detects the connection of the light source module, whether the light source module is connected to the first path or the second path and, when determining that the light source module is connected to the first path, cause the power supplying section to supply the first direct-current power and, when determining that the light source module is connected to the second path, cause the power supplying section to supply the second direct-current power.

Abstract: A light-emitting-diode-driving device includes a control circuit that is configured to perform constant current control with a DC-DC converter so that a value of a current detected by a current detection unit agrees with a prescribed reference current value to be supplied to a light source. The control circuit includes a reference-current-instruction unit, a threshold-voltage-setting unit, and a comparator circuit. The reference-current-instruction unit is configured to set the prescribed reference current value. The threshold-voltage-setting unit is configured to set a threshold voltage for determining a short circuit failure in the light source. The comparator circuit is configured to compare, with the threshold voltage, a value of a voltage that is detected by a voltage detection unit. The control circuit is configured to make the threshold-voltage-setting unit reduce the threshold voltage, when the reference-current-instruction unit reduces the prescribed reference current value.

Abstract: One embodiment of a display backlight driver integrated circuit can be configured for operation in at least two different ways. A first method transfers data from an EEPROM to hardware registers prior to regular operation. A second method also transfers data from an EEPROM to registers. However, hardware registers can be overwritten with data accepted from a control bus, prior to regular operation. A keyboard driver IC can detect the presence or absence of a cable to an LED. If the cable is absent, the driver IC will not supply power for the LED. One embodiment of a keyboard and display backlight control system can be configured to allow substantially independent operation.

Abstract: An LED controller and a method for controlling an LED device is disclosed. A current sense signal is representative of a load current flowing through the LED device. The current sense signal is compared with an upper threshold value and a lower threshold value. Current is provided to the LED device via an inductor coupled in series with the LED device when the current sense signal exceeds the upper threshold. A load current loop is closed over a free-wheeling diode when no current is provided to the LED device while the current sense signal is below the lower threshold. The upper threshold and the lower threshold are adjusted dependent on the current sense signal such that peak values of the current sense signal match corresponding desired peak values.

Abstract: An example power converter includes a first winding, a second winding, a switch, a controller and an output circuit. The second winding is magnetically coupled to the first winding and the controller includes a feedback terminal and a common terminal. The controller is coupled to control the switch to regulate an output of the power converter in response to a feedback voltage received at the feedback terminal. The output circuit is coupled between the common terminal of the controller and a common reference of the power converter to provide an output voltage to a load. The feedback voltage is a positive voltage with respect to the common terminal and the output voltage is a negative voltage with respect to the common reference of the power converter.

Abstract: A dynamic step dimming interface is provided that allows a ballast to energize a lamp in a dim mode or a normal mode. The ballast includes a lamp controller that energizes the lamp using an oscillating current. The oscillating current is also provided to a voltage monitor, which indicates the voltage level of the oscillating current, and to a rectifier, which provides an output indicative of the oscillating current. The rectifier is responsive to user input indicating whether the dim mode or the normal mode is to be used. A processing circuit receives the voltage level from the voltage monitor and provides a mode command to the ballast, indicating the lamp mode, based on inputs received, and provides a reference voltage to a comparator. The comparator receives the rectifier output and the reference voltage, and generates a voltage indicative of a power level of the lamp for the processing circuit.

Abstract: A circuit for driving a light emitting diode (LED) comprises: an alternating voltage power supply, comprising a triac dimmer having a firing angle and output terminals that provide power to the LED; a zero crossing detector that detects a polarity change of the alternating voltage provided by the alternating voltage power supply and provides a zero crossing output signal indicative of the zero crossing; a timer triggered by the zero crossing output signal that generates a timer output signal during a time period of the timer; and LED power circuitry that reduces current to the light emitting diode based upon timing characteristics of the timer output signal.

Abstract: A light emitting diode (LED) illumination device having a power compensation function is provided. The illumination device includes a light emitting unit including a plurality of LEDs connected in series, a rectifier for rectifying input AC power to provide a rectified voltage to the light emitting device, and a power compensator for detecting a change in the rectified voltage provided to the light emitting device and compensating a current provided to the light emitting device according to the detected change in the rectified voltage.

Abstract: An illumination device comprises a solid-state lighting device and a heat sink. The heat sink is configured to be attachable to a fixture for a gas-discharge lamp to retrofit existing gas-discharge fixtures. The heat sink is conductively thermally coupled to the solid-state lighting device to dissipate heat generated by the solid-state lighting device.

Abstract: A power supply circuit for a remote load and a local controller includes a line connection receiving electrical power from an AC source. A load connection connects to the remote load. A switch is located between the line and load connections. Power is supplied to the load from the AC source through the switch. The switch is selectively opened and closed by the controller. A low voltage supply portion supplies power from the AC source to the controller. The low voltage supply portion includes an energy storage device for storing electrical energy for the controller. A current-limited earth ground portion conducts charging current from the energy storage device to earth and prevents charging current conducted to earth from exceeding a predetermined current level. An earth ground bypass portion conducts at least some of the charging current to the load when the switch located between the line and load is open.

Abstract: A driver circuit for driving light emitting diodes (LEDs). The driver circuit includes a string of LEDs divided into n groups and the n groups of LEDs is electrically connected to each other in series, where a downstream end of group m?1 is electrically connected to the upstream end of group m. The driver circuit also includes a power source coupled to an upstream end of group 1 and provides an input voltage. The driver circuit further includes current regulating circuits, where each of the current regulating circuits is coupled to the downstream end of the corresponding group at one end and coupled to a ground at the other end. Each of the current regulating circuits includes a sensor amplifier and a cascode having first and second transistors. The driver circuit also includes detectors, where each of the detectors detects a source voltage of the first transistor.

Abstract: Provided is a step up/down converter, in which the inductor current can be easily detected by a simple configuration in any connecting mode of the switches.

Abstract: A driver circuit for a lighting apparatus includes a current regulator configured to supply a load current to a load, and a control circuit coupled to the current regulator and configured to receive a dimming control signal and to linearly vary an amplitude of the load current in response to the dimming control signal.

Abstract: A light-emitting diode (LED) lighting fixture is configured to interface a three-way socket and provide three levels of output light intensity. A detection unit determines whether one or both input voltages lines of the three-way socket are active and provides a control signal to an LED controller indicating which input voltage lines are active. The LED controller is configured to provide different output current levels to an LED bank based on whether one or both input voltage lines are active. By adjusting the output current level based on which input lines are active, the LED lighting fixture provides three output lighting intensity levels. LED lighting fixtures according to various embodiments can therefore serve as a direct replacement for three-way incandescent bulbs.

Abstract: There is provided a light emitting diode power supply apparatus capable of limiting a rise in a voltage level of a power supplied to a light emitting diode even in the case in which driving of the light emitting diode is stopped for a long period of time. The light emitting diode power supply apparatus includes: a power supply unit supplying a driving power set under a control to a light emitting diode; and a controlling unit selecting one of a detection voltage obtained by detecting a current flowing to the light emitting diode and a voltage provided according to an operation state of the light emitting diode to control a power supply operation of the power supply unit.

Abstract: A converter, for feeding a load via an inductor, includes a switch to permit or prevent the feeding of current towards inductor, and a current sensor with a resistor coupled to the converter output, adapted to sense the current through said inductor when switch is off. A further switch is provided which is conductive when said switch is turned off; moreover, a drive circuitry is provided which is coupled to current sensor to turn said switch on as a function of the detected current. Drive circuitry is fed by a bootstrap circuit which includes: a bootstrap capacitor to accumulate a feeding charge for drive circuitry and coupled to the converter output and to a bootstrap diode, a coupling capacitor interposed between said further switch and bootstrap diode, as well as a bootstrap resistor interposed between a power supply source and bootstrap diode to charge coupling capacitor therefrom.

Abstract: Driver circuit and driving method for an LED lighting device are provided. The driver circuit includes an AC power supply, a dimmer, a transformer, and/or a dimming control circuit. The dimming control circuit includes a filtering unit, a rectifying unit, a boost converter unit, a control-signal conversion unit, and/or a buck converter unit. A dimming position is defined at a point in the driver circuit to perform a dimming control process of the LED lighting device by the dimmer. A voltage corresponding to the point is boosted by the boost converter unit to provide a boosted voltage. The boosted voltage is sampled and converted by the control-signal conversion unit into an analog control voltage signal acceptable by a buck converter unit. The buck converter unit alters an amount of a current of the LED lighting device, after receiving the analog control voltage signal.

Abstract: Technologies are described herein for an illumination device and a corresponding fixture device. The illumination device includes a luminary module for the emission of light and an identification circuit containing identifying data, while the fixture device includes a driver module for supplying power to the illumination device and a controller module. When the illumination device is connected to the fixture device, the controller module communicates with the identification circuit of the illumination device to retrieve the identifying data and causes the driver module to supply the appropriate power to the luminary module of the illumination device.

Abstract: In a conventional example, even if a duty cycle of the burst dimming is changed during an OFF-period of a switching element, current flowing to an LED is maintained constant. On the other hand, in the present embodiment, an accumulated value of ON-periods of a switching element is increased or decreased so as to be linked to a minimum variation width for a duty cycle (a dimming level) of a dimming signal, regardless of a timing of when the duty cycle is changed. Therefore, a lighting system (an LED drive device) according to the present embodiment can change smoothly a light output of a solid-state light-emitting element (a light source) with respect to a change in a duty cycle of the burst dimming while preventing the switching frequency from increasing.

Abstract: A two-wire smart load control device, such as an electronic switch, for controlling the power delivered from a power source to an electrical load comprises a relay for conducting a load current through the load, a controller for rendering the relay conductive and non-conductive, and an in-line power supply coupled in series with the relay for generating a supply voltage across a capacitor when the relay is conductive. The power supply controls when the capacitor charges asynchronously with respect to the frequency of the source. The capacitor conducts the load current for at least a portion of a line cycle of the source when the relay is conductive. The controller is operable to determine when the magnitude of the supply voltage reaches a maximum supply voltage threshold, and render the relay non-conductive immediately after the supply voltage reaches the maximum supply voltage threshold.

Abstract: A power supply or driver circuit configured to provide electrical energy at a drive voltage. The driver circuit converts electrical energy at an input voltage to the electrical energy at the drive voltage. A controller is configured to control the power converter to provide electrical energy at the drive voltage. The controller stops operation at an interruption of electrical energy to the driver circuit. The controller is configured to resume operation subsequent restoration of electrical energy to the driver circuit. The controller is configured to maintain the timing voltage above a first voltage level when the controller is in operation and to determine the duration of an interruption of electrical energy to the driver circuit.

Abstract: In one general aspect, a dimmer power supply circuit is disclosed for use with single-pole dimmers that includes a first AC mains input line, second AC mains input line, and a rectification circuit responsive to the first and second AC mains input lines. A positive LED supply terminal responsive to the rectification circuit, and a negative LED supply terminal is responsive to the rectification circuit. At least one preload resistor is operatively connected in a first intermittent loading circuit path between two supply rails in the dimmer power supply circuit, and a peak detector is responsive to the first and second AC mains input lines.

Abstract: Apparatus and associated methods reduce harmonic distortion of a excitation current by diverting the excitation current substantially away from a number of LEDs arranged in a series circuit until the current or its associated periodic excitation voltage reaches a predetermined threshold level, and ceasing the current diversion while the excitation current or voltage is substantially above the predetermined threshold level. In an illustrative embodiment, a rectifier may receive an AC (e.g., sinusoidal) voltage and deliver unidirectional current to a string of series-connected LEDs. An effective turn-on threshold voltage of the diode string may be reduced by diverting current around at least one of the diodes in the string while the AC voltage is below a predetermined level. In various examples, selective current diversion within the LED string may extend the input current conduction angle and thereby substantially reduce harmonic distortion for AC LED lighting systems.

Abstract: The invention provides a plurality of individually addressable radio frequency (RF) modules, any of which can be installed with any electrical device such as an ambient condition sensor or an ambient condition modifier. A prime example would be a light fixture, whether with or without a dimmable light source efficient dimming and integrated smart sensor networking related to the lighting system itself or to other systems such as parking monitors, fire alarm monitors or security alarm monitors. Independent control processing in each lighting fixture or electrical device allows a multiplicity of sensors to be locally employed and their data to control local conditions or communicate to adjoining fixtures and electrical devices and thereby control larger portions of the lighting system network or network to several unrelated systems.

Abstract: A light source apparatus for an endoscope, which has a lamp, a power source, a light adjustment control module that controls power to be supplied to the lamp within a range not more than an upper limit power based on a brightness instruction signal, and a continuous lighting sensing module that measures a continuous lighting time period from time when the lamp shifts from being extinguished to being lighted, compares the continuous lighting time period with a predetermined upper limit time period, and initializes the continuous lighting time period while the lamp is extinguished, wherein the upper limit power is set to a first upper limit power when the continuous lighting time period has not reached the upper limit time period, and changed to a second upper limit power lower than the first upper limit power in a stepwise manner when the upper limit time period has not been reached.

Abstract: This document discusses, among other things, an apparatus, system, and method to limit a current overshoot in an electronic component using a switched feedback circuit to precondition a gate of a transistor coupled to the electronic component.

Abstract: A method for operating a HID lamp using a bi-power electromagnetic ballast. The method includes applying a full power level to the lamp to produce full brightness and applying a dimmed power level to the lamp to produce a dimmed brightness. The lamp may be dimmed by switching the lamp power from the full power level to the dimmed power level and then applying a full power pulse to the lamp. The dimmed power level is less than the full power level.

Abstract: A voltage regulation circuit includes a voltage conversion module, an LED driving module and a control module. The voltage conversion module is operable to receive and convert the first voltage to a second voltage. Each LED driving loop of the LED driving module has a current regulator and an LED string. The control module is operable to output a control signal according to the received input signal which reflects the current flowing through the LED driving module. The voltage conversion module regulates the second voltage according to the control signal, such that the regulated second voltage still keeps the brightness of the LED string of each LED driving loop unchanged.

Abstract: There is provide a light emitting diode lighting apparatus capable of being generally used in various kinds of dimmer, the apparatus including: a dimmer varying waveforms of input power according to a dimming signal; a bleeder adjusting a bleeding current under control; a controller comparing a latching current of the dimmer with the bleeding current of the bleeder to control the amount of the bleeding current; and an LED driver receiving a current other than the bleeding current flowing through the bleeder from among the currents from the dimmer to driver a light emitting diode (LED).

Abstract: The present invention discloses methods and devices for transmitting/obtaining identification information and positioning by visible light signal.

Abstract: A lighting system includes methods and systems to mix colors of light emitted from at least two LED emitters. In at least one embodiment, the lighting system includes a controller that responds to phase-cut angles of the dimming signal and correlates the phase-cut angles with a predetermined black body radiation function to dynamically adjust a color spectra of the mixed light in response to changes in phase cut angles of the phase-cut dimming level signal. In at least one embodiment, the controller utilizes the predetermined black body radiation function to dynamically adjust the color spectra of the mixed, emitted light in response to changes in phase cut angles of a phase-cut dimming level signal. In at least one embodiment, the predetermined black body radiation function specifies correlated color temperatures (CCTs) that model the CCTs of an actual non-LED based lamp, such as an incandescent lamp.

Abstract: The present disclosure relates to lighting fixtures for use in a lighting network where the lighting fixtures and other elements are able to communicate with each other via wired or wireless communication techniques. When the lighting network is being formed or modified, a lighting fixture is selected to act as a coordinator for forming the lighting network. For example, a user may employ a commissioning tool to select a particular lighting fixture as the coordinator. The coordinator will send out one or more ‘join my network’ messages toward the other elements of the lighting network. The elements that receive the ‘join my network’ message may respond in order to make the coordinator aware of their presence and join them to a lighting network.

Abstract: A photoswitch includes a DC power supply, a luminance controlling circuit and a lightening apparatus. The luminance controlling circuit includes a relay RL1, a trigger circuit module, a delay discharge circuit module, a photosensitive element and an adjusting circuit module. The trigger circuit module includes a NPN triode Q1, a NPN triode Q2, NPN triode Q3, a resistor R3 and a resistor R6; the adjusting circuit module includes a resistor R4 and a resistor R5 and a resistor R7, one end of which is in connection with the current output end of the resistor R4 and the other of is in connection with the current input end of the relay RL1; a current value of a subcircuit with the resistor R7 and the relay RL1 is smaller than a pull-in current value or a release current value of the relay RL1.

Abstract: An LED backlight driving circuit has a voltage converting unit having an output terminal connected to a positive electrode of an LED to provide a driving voltage required by the LED, a driving unit to control the voltage converting unit to achieve a voltage conversion, and a protection unit connected in series between a negative electrode of the LED and the driving unit. A maximum allowable power of the protection unit is less than a power when the driving voltage of the LED is directly applied to the protection unit. The circuit also includes a control unit for outputting a control signal for stopping working to the driving unit when the protection unit forms an open circuit. Thus, the circuit parts can be protected, and the short-circuit protection function is more safe and reliable.

Abstract: A lighting device formed of a FIPEL panel driven by electrical connection. For example, a frequency generator can create a frequency that creates a light output having any frequency in the spectrum. The light emitting panel can be flexible, and can be coded along a curved surface, such as the inner surface of a light bulb.

Abstract: A backlight module and a liquid crystal display (LCD) device thereof are proposed. The backlight module includes a DC/DC converter producing a positive/negative voltage level. The backlight module can output the positive and negative levels of driving voltage from an output end of the inverter to drive LEDs through alternately switching. Besides, a constant current can be obtained with designs of power on/off periods and energy-storing inductors to prevent LEDs employing current limiting resistors from having too low current efficiency and from being burnt out.

Abstract: A method for driving light adjustments, a device therefor and a light adjustable lamp including the device are disclosed. The device includes a light source driving circuit, an MCU and a temporary power supplying circuit. The MCU includes a current detecting module, an analyzing and processing module and a storage module. The analyzing and processing module obtains a connecting signal or a disconnecting signal of the light source driving circuit and thereafter extracts a stored firmware program of gradually varying brightness of a light source or stored information about a constant brightness state of the light source from the storage module for analyzing and processing, so as to generate a controlling signal of adjusting the brightness of the light source and send the controlling signal into the light source driving circuit. Then the light source driving circuit adjusts the brightness of the light source according to the controlling signal.

Abstract: An LED controller is disclosed herein. An embodiment of the controller includes a first input connectable to a power source and an output connectable to at least one light-emitting diode (LED). A power factor correction circuit is coupled between the first input and the output, wherein the power factor correction circuit operates in a first state when the power factor is corrected and wherein the power factor correction circuit operates in a second state when the power factor is not corrected. The power factor correction circuit is in the first state when no dimming of the LED is sensed, and the power factor correction circuit is in the second state when dimming of the LED is sensed.

Abstract: The present invention relates to a high efficiency LED driver and driving method thereof. In one embodiment, a high efficiency LED driving method configured for a LED device can include: (i) receiving a DC bus voltage and generating a driving voltage for the LED device through a power switch; (ii) comparing the DC bus voltage against a sum of the driving voltage and a first reference voltage; (iii) where when the DC bus voltage is greater than the sum of the driving voltage and the first reference voltage, generating a first output current; (iv) where when the DC bus voltage is greater than the driving voltage and less than the sum of the driving voltage and the first reference voltage, generating a second output current; and (v) matching an average current of the first output current and the second output current with a corresponding driving current.

Abstract: Phase angle detection techniques for phase-cut dimming lighting circuitry are disclosed. A phase-cut lighting driver circuit may include galvanic isolation circuitry having a primary and secondary side. The phase angle information of a phase-cut signal may be detected on the secondary side of the driver circuitry, and a microcontroller can create a dimming signal that adjusts the driver output power according to the phase angle information. In some embodiments, the phase angle detection techniques may be utilized to control the output of lighting driver circuitry, such as a phase-cut dimming LED driver.

Abstract: A LED current control system for use with an LED drive system which includes LED strings connected in series with respective current sink circuits, each of which causes a current to be conducted by the LED string to which it is connected. The drive system includes 3 or more ‘dimming’ inputs with which the LED string currents can be adjusted. The LED current control system comprises at least one minimum circuit which receives two or more dimming inputs and produces an output which is proportional to the lesser of the inputs, a multiplier circuit which receives the outputs of the minimum circuits and at least one other dimming input and produces an output ILED which is proportional to the product of the received signals, and a sink control circuit which receives ILED and controls the current sink circuits such that the string currents vary with ILED.

Abstract: Various embodiments of the present invention relate to a LED illumination system, and more particularly, to systems, devices and methods of employing a LED driver control loop to adjust the brightness of the LED light smoothly and suppress flickering/blinking. The LED driver control loop comprises a LED driver, a charge integrator, a VPWR modulator, and a driver controller. The charge integrator generates a voltage VC that is associated with a LED current iLED and illumination energy for the LED light. The VPWR modulator provides a clamping voltage VPWR such that the LED driver ceases to inject the LED current iLED as the voltage VC saturates at VPWR during each powering cycle. The clamping voltage VPWR is dynamically adjusted at the end of each powering cycle to gradually adjust the brightness and avoid flickering or blinking while still ensuring illumination efficiency.

Abstract: Strings of light-emitting elements are driven by providing a fixed voltage reference to a plurality of parallel-connected current sources, each of the current sources being connected in series with a string, supplying current from each current source to the string to which it is connected, whereby the fixed voltage reference is independent of the current supplied to each string, and taking up excess voltage not required by any one of the strings of light-emitting elements.

Abstract: A method of driving a light source includes receiving an alternating current (“AC”) voltage from outside, generating a first direct current (“DC”) voltage based on the AC voltage, generating a second DC voltage, corresponding to a difference between a driving voltage of a light source part and the first DC voltage, based on the first DC voltage, and outputting a sum of the first DC voltage and the second DC voltage to the light source part.

Abstract: An LED driving device includes: a rectifying circuit for outputting a DC voltage to a string of M LED units; (M?1) first switching circuits each coupled between a corresponding one of first to (M?1)th LED units and ground; and a second switching circuit coupled between an Mth LED unit and ground. When the DC voltage is sufficient to turn on first to kth LED units, where 1?k?M, the kth LED unit is coupled to ground through first and second conductive paths provided by a resistor unit, and a corresponding first switching circuit or the second switching circuit, and each of the first to (k?1)th LED units is coupled to ground through a third conductive path provided by a corresponding first switching circuit and the resistor unit.

Abstract: A circuit includes an input to be coupled to receive a rectified line voltage having a controlled conduction phase angle in each half line cycle. An active device is coupled to a feedback terminal of a controller. The feedback terminal is coupled to receive a feedback signal representative of an output of a power supply. The active device includes a control terminal coupled to receive a signal representative of the input. The active device is coupled to adjust the feedback signal on the feedback terminal in response to the control of the conduction phase angle of the rectified line voltage in each half line cycle.